Improved carrier transport in Mn:ZnSe quantum dots sensitized La-doped nano-TiO₂ thin film

Mn:ZnSe/ZnS/L-Cys core-shell quantum dots (QDs) sensitized La-doped nano-TiO₂ thin film (QDSTF) was prepared. X-ray photoelectron spectroscopy (XPS), nanosecond transient photovoltaic (TPV), and steady state surface photovoltaic (SPV) technologies were used for probing the photoelectron behaviors in the Mn-doped QDSTF. The results revealed that the Mn-doped QDSTF had a p-type TPV characteristic. The bottom of the conduction band of the QDs as a sensitizer was just 0.86 eV above that of the La-doped nano-TiO₂ thin film, while the acceptor level of the doped Mn²⁺ ions was located at about 0.39 eV below and near the bottom of the conduction band of the QDs. The intensity of the SPV response of the Mn-doped QDSTF at a specific wavelength was ～2.1 times higher than that of the undoped QDSTF. The region of the SPV response of the Mn-doped QDSTF was extended by 191 nm to almost the whole visible region as compared with the undoped QDSTF one. And the region of the TPV response of the Mn-doped QDSTF was also obviously wider than that of the undoped QDSTF. These PV characteristics of the Mn-doped QDSTF may be due to the prolonged lifetime and extended diffusion length of photogenerated free charge carriers injected into the sensitized La-doped nano-TiO₂ thin film.